Flexible filter sheet materials are needed for a wide variety of technical purposes, such as aircraft construction, medical apparatus, water purification and air conditioning equipment, to prevent the passage of toxic or harmful vapors and gases. Another important field of application is protective clothing and the like, which as a rule contains a layer of material for absorbing chemical poisons. Typical examples are polyurethane foams and mats charged with a mixture of finely ground active carbon and a binder dispersion. These known flexible filters have the great disadvantage that they are washable only to a limited extent and, on account of their laminated construction, they impair the exchange of air required by the skin and consequently result in a build-up of heat. If foam is used as the supporting material, there is an irregularity of coverage due to the pores of the foam and hence an irregularity of the filtration effect which then varies widely.
German Pat. No. 2,951,827 discloses a material for protection against chemicals and brief exposure to heat, which consists of an air-permeable, flexible supporting layer bearing on one or both sides grains of adsorbent of a diameter of approximately 0.5 mm on pillars of a hardened adhesive. These permeable sheet materials satisfy a great number of requirements, but they are difficult to make and, on account of the granular adsorbent they require, e.g., finely granular active carbon, they are relatively expensive. In many cases, irregular charging causes the adsorber particles to contact one another in movement, which results in attrition.
Sheet filter materials made from carbonized fabric are extremely expensive and cannot withstand mechanical stress. Combining them with other supports is of no more than limited usefulness.